1 Technical Field
The present invention relates to rotating electric machine drive systems that are used in, for example, motor vehicles to drive electric motors and electric generators.
2 Description of Related Art
Conventionally, it has been desired for electric motors for electric vehicles to exert high-torque and high-efficiency characteristics over a wide operating range from low speed to high speed.
To satisfy such a desire, there has been developed a method of switching the connection of a stator coil of an electric motor between a Y connection (or star connection) and a Δ connection (or delta connection) so as to enable the electric motor to exert optimal characteristics in both a low-speed region and a high-speed region (see, for example, Japanese Patent Application Publication No. JP2014054094A).
When the electric current, which is supplied from a drive apparatus to the stator coil of the electric motor, includes third-order and (3+6n)th-order harmonics, the iron loss of the electric motor is increased. The increase in the iron loss is not desirable. Therefore, according to the above method, the A connection is used in the high-speed region, whereas the Y connection is used in the low-speed region.
Accordingly, in a low-speed and low-torque region, the Y connection is used. However, in the low-speed and low-torque region, it has not been possible to improve the torque characteristics of the electric motor with the Y connection.
On the other hand, in recent years, it has been desired for rotating electric machines used in motor vehicles to have a high efficiency particularly in a low-speed and low-torque region, where the rotational speed is in the range of 1000 to 5000 rpm, when the motor vehicles travel in urban areas.